關於com.google.android.maps.MyLocationOverlay 問題

		MapView mapview = (MapView) findViewById(R.id.map_mymap);

		mapview.setClickable(true);

		mapview.setEnabled(true);

		mapview.setBuiltInZoomControls(true);	



		MapController controller =  mapview.getController();		

		controller.setZoom(16);



		MyLocationOverlay myoverlay = new MyLocationOverlay(this,mapview);

	

		myoverlay.enableMyLocation();

	

		mapview.getOverlays().add(myoverlay);

這個MyLocationOverlay 加上去一點作用也沒有。
照官方文檔上講應該至少會給我畫一個 藍色的小圓點出來呀，什麼都沒有？

Draw the "my location" dot. By default, draws an animated "blue dot" asset, possibly surrounded by an outlining blue disk to represent accuracy. Also, if the user's position moves near the edge of the screen, and we've been given a MapController in our constructor, we'll scroll to recenter the new reading.

難道要我重寫 MyLocationOverlay 類嗎？
要是這樣的話，那跟 重寫 ItemizedOverlay 有什麼區別啊。鬱悶中。。。。